Hi Matt;
Old Cadillac tail light lenses from the 50's had the same problem, the sun would bleach out all the color, so we resorted to restoring them with paint as stated below.
Sedan Tail lights have color in the plastic as you can see when you remove them. Bringing the color back to the surface will take some time, effort and care not to cause damage to the high spots. The 2000 grit idea is good but not aggressive enough to bring out quick results and could cause you problems later. I start with 800 grit and use Mothers mag wheel polish to make the first cut. Then go to lighter polishes and 1000 grit to help smooth the process. Pay attention to the corners and edges and around the letters, it will help with the looks. But if the sun has done its best to bleach out more "Red" than we thought, it's best to stop polishing as the plastic is only so thick.
Next step. For the best look, go to the hobby shop and pick up some candy red (no metallic needed here and the candy will allow the light to shine through the lens) and some clear spray paint (lacquer if you can get it and if you have lacquer red, then the clear also has to be Lacquer). Stop by your local "Professional" car paint dealer and pick up a pint of Grows Super Clean (or like product). The super clean is used to remove all the wax and other contaminants that will make the paint show fish-eyes and other ugly stuff. After the sanding and polishing, your lenses should look clean and if there are any scratches, they should be smoother than they were or gone. Now, clean the lenses completely, even a thumb print here will cause problems. The wag you use should be clean, dry (not the one you used to polish your lenses) because using the super clean with a contaminated cloth will only put more problems in to the mix. Clean them twice and read the directions.
Place the lenses about 18 inches apart and on a 2X4 so the edges are not touching paper... In a clean, dry, well ventilated area with temperatures at a about 70 degrees, (observe all the directions on the spray paint can) spray on a dry coat. This will be a quick all over the lenses spray with the red. leave them for 15 minutes so the paint will flash. It will also help to keep the fish eyes from forming (if your cleaning job was not that clean) and help keep runs out of the picture. After 15 minutes, apply a second coat, if the directions on the can says hold the can 9 inches away then keep the can 9 inches away.
The second coat will be a wet coat (more paint by going slower in the application) again watch the paint so runs don't form. And wait another 15 minutes for the paint to flash before another or final coat. If two coats are enough then leave it with two coats. The issue here is to have them red, but not to much paint so as to block the light from coming through. Observing the 15 minute flash rule apply a coat of clear, wait for flash time and apply another coat. Let them dry over night, the build up of clear over the red will allow color sanding and polishing (if you used lacquer).
Remember, if you can't find lacquer then you really have to read the directions as the process explained above will work for lacquers and enamels have to dry for a week or so before any sanding but all bets are off on anything else. And, talk to the professional paint dealer for more ideas if lacquer is out of reach.
Bill